The present invention relates, in general, to a system and method for tracking a target. More particularly, the present invention relates to a method for passively determining range from a moving platform to an emitter, wherein the emitter is the target.
Target acquisition and tracking are important functions in the desire for autonomy in system design of a vehicle. Vehicles are often required to sense their environment and track targets that are crucial to their navigation profile. Target state estimation is required to provide or predict an accurate target state from a variety of sensors onboard the vehicles.
A conventional method for producing range to a target with respect to a radio wave (RF) signal is to use an active sensor onboard the vehicle. Typical active sensors are radio or acoustic radar or laser range finder sensors. An active sensor transmits a signal to a target. The signal is then reflected off the target and received by the active sensor. The range to the target is calculated by a processor onboard the vehicle based on the travel time between the active sensor transmitted signal and the target reflected signal.
Passive tracking methods offer significant advantages over active tracking methods. Unlike radar, laser and other active tracking sensors, passive sensors do not emit any kind of energy. They only receive target emitted energy and transform the energy for measurement purposes. This characteristic makes passive tracking methods an ideal technique in reconnaissance and surveillance applications. Passive tracking methods can detect the target and, at the same time, can keep the detecting platform or vehicle hidden from any external detection by the target, as it emits no signals.
In general, however, a passive tracking sensor cannot measure range or distance between the sensor and the target, as it is not based on the echoed-signal principle. The passive sensor, typically, offers only measurement of the target direction with respect to the received RF carrier in space. It is, therefore, very challenging to estimate accurate range information to a target from a passive sensor. The present invention addresses the challenge of estimating accurate range and angle information to a target using only passive sensors onboard a moving vehicle or platform.
To meet this and other needs, and in view of its purposes, the present invention provides a method of determining range from a moving platform to an emitter. The method includes the steps of: (a) receiving a RF signal from the emitter; (b) counting a number of phase reversals of the received RF signal during a period of time; (c) measuring a Doppler frequency during the period of time; and (d) determining the range to the emitter based on both the number of phase reversals counted in step (b) and the Doppler frequency measured in step (c). Step (b) includes counting the number of phase reversals of the received RF signal during the period of time the moving platform traverses a distance. The method may further include the step of: (e) measuring the distance traversed by the moving platform during the period of time. Step (d) may include determining the range to the emitter based on the number of phase reversals counted in step (b), the Doppler frequency measured in step (c) and the distance measured in step (e). Measuring the distance may include obtaining geographic position data at each end of the distance traversed by the moving platform, using either an inertial navigation system (INS), a Global Positioning System (GPS), or a combination of an INS and GPS.
Another embodiment of the invention provides a method of determining range from a moving platform to an emitter. The method includes the steps of: (a) receiving a RF signal from the emitter during a period of time the moving platform traverses a distance, the distance denoted by b; (b) determining a carrier wavelength, xcex, of the RF signal; (c) counting a number of phase reversals of the received RF signal during the period of time, the number denoted by N; (d) determining a range differential, xcex94R, between the moving platform and the emitter during the period of time, in which xcex94R=Nxcex; (e) measuring a Doppler frequency, fd, during the period of time; and (f) determining the range to the emitter based on the distance b, the range differential xcex94R and the Doppler frequency fd.
Yet another embodiment of the invention provides an apparatus, installed onboard a moving platform, for determining range from the moving platform to an emitter. The apparatus includes a receiver for receiving a RF signal from the emitter, an analog to digital converter (ADC) for converting the received RF signal into a digital signal, a memory for storing the digital signal provided by the ADC, a processor coupled to the memory for extracting the stored digital signal, and (a) counting a number of phase reversals of the digital signal during a period of time, (b) measuring a Doppler frequency during the period of time, and (c) determining the range to the emitter using both the counted number of phase reversals and the measured Doppler frequency.
The apparatus may also include a GPS receiver coupled to the processor for obtaining geographic position of the moving platform, and the processor determining a distance traversed by the moving platform during the period of time based on the geographic position obtained from the GPS receiver. The apparatus may also include a mixer coupled between the receiver and the ADC for converting the received RF signal into an intermediate frequency (IF) signal, where the ADC converts the IF signal into the digital signal. The mixer may be coupled to a numerically controlled oscillator (NCO) for providing a coherent signal to the mixer, and the mixer may combine the received RF signal and the coherent signal to provide the IF signal.
It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.